Field of the Invention
The present invention relates to a humidification device for humidifying process gases, particularly for fuel cells, and to a fuel cell arrangement comprising such a humidification device.
Description of the Background Art
Fuel cells utilize the chemical reaction of a fuel with oxygen to form water in order to produce electrical energy. For this purpose, fuel cells contain as the core component the so-called membrane electrode assembly (MEA), which is a composite of a proton-conducting membrane and two electrodes (anode and cathode) enclosing the membrane in a sandwich-like manner. The fuel cell is typically formed by a plurality of MEAs arranged in a stack, with their electrical power being additive. During the operation of the fuel cell, the fuel, particularly hydrogen H2 or a hydrogen-containing gas mixture, is supplied to the anode, where an electrochemical oxidation takes place with the release of electrons (e.g., H2→2 H++2 e−). A (water-bound or water-free) transport of protons H+ from the anode compartment to the cathode compartment occurs across the membrane, which separates the reaction chambers from one another in a gas-tight manner and isolates them electrically. The electrons provided at the anode are supplied to the cathode via an electrical circuit. The cathode is supplied in addition with oxygen or an oxygen-containing gas mixture, so that a reduction of oxygen occurs with the uptake of electrons (½O2+2 e−→O2−). At the same time, in the cathode compartment the formed oxygen ions react with the protons transported across the membrane with the formation of water (O2−+2 H+→H2O). Fuel cells achieve an improved efficiency by the direct conversion of chemical energy into electrical energy compared with other electricity generators because of the circumvention of the Carnot factor.
A focus of current fuel cell development is directed particularly to traction applications for the propulsion of motor vehicles. The currently most developed fuel cell technology is based on polymer electrolyte membranes (PEM), in which the membrane is made from a humidified polyelectrolyte (e.g., Nafion®) and the water-bound electrolytic conduction takes place via hydrated protons. Such polymer electrolyte membranes for proton conduction are dependent on the presence of water. Below a certain temperature, the cathodically formed product water can still be sufficient here as a humidity source for humidifying the membrane. However, at higher temperatures moisture is taken increasingly out of the fuel cell stack with the cathode exhaust gas. To counter a drying out of the fuel cell membrane here, the removal of moisture must be compensated by the active supplying of water.
WO 98/45889 A1 describes a fuel cell with an internal water supply, in which water is added in the form of an aerosol both to the fuel gas in the anode area and to the air in the cathode area. The introduction occurs via channels in the specific bipolar plate.
In addition, DE 11 2005 000 819, which corresponds to U.S. Pat. No. 7,241,474, discloses providing gas diffusion media within the fuel cell, which have hydrophobic and also hydrophilic properties or regions and thus adjust the water balance of the cell by suitable supplying and removal of moisture.
It is known, furthermore, to use external humidification devices in order to humidify the process gas to be supplied to the fuel cell, in most cases the air to be supplied to the cathode compartments. In this regard, in particular a portion of the moisture removed from the stack with the waste air of the cathode compartments is returned. The strategy of the moisture recirculation is realized for PEM fuel cells either by means of the diffusion of water across water vapor-permeable membranes and/or according to the capillary principle through very fine channels in a porous layer. So-called hollow fiber modules are suitable for the diffusion and capillary principle. Membrane humidifiers utilize the product water formed by the fuel cell reaction at the cathode during use of a water vapor-permeable membrane, in order to humidify the process gas to be supplied to the fuel cell. Not only is the drying out of the membrane prevented in this way but also an excessive accumulation of water in the fuel cell.
US 2008/0241636 A1 (DE 10 2008 016 087 A1) describes such a membrane humidifier, which is formed according to the principle of a counterflow heat exchanger, whereby a water vapor-rich gas is passed through conduits which are enclosed by a housing through which the gas mixture to be humidified flows in a counterflow. The conduits has a water vapor-permeable membrane material.
DE 10 2009 005 685 A1, which corresponds to U.S. Pat. No. 8,051,992, discloses an external membrane humidification device, which has a stack of corrugated plates each with a membrane disposed therebetween. Flow channels, through which some of the relatively moist cathode exhaust gas flows and some of the cathode air to be humidified as well, are formed by the corrugated plates. The membrane is contacted on both sides by a layer of a hydrophilic diffusion medium, which, on the one hand, is to take up water and to transport it to or from the membrane and, on the other, is to support the membrane structurally.
US 2009/0092863 A (DE 10 2008 050 507 A1) describes a membrane humidifier, which has a stack of alternating wet plates and dry plates, between which a water vapor-permeable membrane is arranged. Each wet and dry plate includes two gas diffusion layers, between which lands are arranged that define the flow channels. The flow channels of the wet plate conduct wet exhaust air from the cathode side of the fuel cell and the flow channels of the dry plate conduct relatively dry process gas that is supplied to the fuel cell. The plates are sealed laterally by massive plastic strips.
The membrane humidifiers disclosed in US 2008/0001313 A1 correspond to the previously described humidifier but have different channel structures. Here the channels of the wet and dry plates are produced by plates with a corrugated sheet structure or by plates with grooves on both sides, instead of by lands.
The prior-art external humidification devices for fuel cells when using water vapor-permeable membranes have in common that the membrane is always enclosed on both sides by layers of a diffusion medium, particularly a nonwoven made of glass fibers or a plastic, e.g., polyether ketone (PEEK), polyether imide (PEI), or polysulfone (PSU). The diffusion layers have transport functions for the water vapor by means of convection and a support function for the membrane. A disadvantage of the use of such diffusion layers in the form of nonwovens, on the one hand, is the relatively large layer thickness of the nonwoven that is typically within the range of 200 μm. Because of the high number of diffusion layers present in a humidification device, this leads, on the one hand, to long flow paths through the diffusion layer and, on the other, to a high installation space requirement, which particularly in vehicles is often not available. Moreover, nonwovens have a relatively small free area of typically less than 50%, which leads to a reduced water transport rate.